Myxococcus xanthus represents an excellent model system to address fundamental questions of how cell-cell signaling pathways control multicellular development. These questions are relevant to all normal embryonic and adult cells that transduce signals to coordinate processes such as growth and differentiation, as well as to cells that are defective in signaling networks, such as cancer cells. Progression through early multicellular development requires that M. xanthus cells sense and respond to a high cell density and nutrient limitation. Two sensitive sensing networks monitor these extracellular signals and converge at a critical checkpoint early in M. xanthus development. This check -point can be monitored by expression of a specific gene, spi. The long-term goals of this research are to determine: i) How do the cells sense and transduce the cell-density signal? ii) How are the cell-density- and nutrient-sensing pathways integrated? iii) What is the connection between the change in gene expression and the complex behavioral response of multicellular fruiting body formation? The identification and characterization of three critical regulators of spi expression, SasS, SasR, and SasN has begun to address these questions. Using classical and molecular genetics combined with protein biochemistry, they have generated a hypothesis for the mechanism by which these proteins integrate both the cell-density signal (extracellular A signal) and the starvation signal, creating the circuitry that controls the developmental expression of the responsive spi gene. To test this hypothesis, they plan to: i) analyze the SasS/SasR/SasN-dependent integration of cell density and starvation signals during early M. xanthus development, ii) generate an in vitro transcription system to test the activity of the SasS/SasR pathway, iii) analyze the stimulation of the SasS/SasR pathway by alterations in cell-surface integrity, and iv) investigate SasS/SasR-independent A signal transduction pathways. Their ultimate goal is to reconstitute the complete signaling pathway as proof of its function.